1. Field of the Invention
The invention generally relates to the transmission of signals in communication systems. More specifically, the invention relates to systems and methods of mitigating the effects of dispersion in communication systems.
2. Background Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through a fiber-optic cable using light emitting diodes or lasers. To send data on a fiber-optic cable, the data is typically converted from electronic data generated by computers to optical data that can be propagated onto the fiber-optic cable. This conversion is generally done using a laser or light emitting diode. When light is generated (or is at “high power”) a logical “1” is presented. The absence of light (or light at “low power”) represents a logical “0”. Thus an optical signal with sharp rising transitions and falling transitions representing logical “1” and “0” bits is transmitted onto the fiber-optic cable.
Much of the fiber-optic cable presently installed is multi-mode fiber, such as a multi-mode fiber 100 shown in FIGS. 1 and 2. As shown in FIGS. 1 and 2, the multi-mode fiber 100 typically includes a core 102 surrounded by a cladding 104.
With multi-mode fiber, signals travel through different paths along the multi-mode fiber 100. For example, one or more portions 106 of a signal may travel directly down the multi-mode fiber 100 while other portions 108 of the signal “bounce” or are reflected back and forth along the fiber-optic cable. For portions of the signal that are reflected back and forth, each portion may take a different path resulting in different velocities at which the portions of the signal travel through the multi-mode fiber 100.
When a single signal takes several paths as it travels along the multi-mode fiber 100, the signal may disperse, which may cause portions of adjacent bits to disperse into each other (commonly referred to as “intersymbol interference”). For example, portions of a “1” or high power bit may spread into the time of the signal previously occupied by a “0” or low power bit. Similarly, the absence of power in a “0” or low power bit may cause a decrease in the power of an adjacent “1” or high power bit at various times within the time of the signal previously occupied by a “1” bit. Signal dispersion is more severe as the distances that the signals travel is increased and also as the frequency at which the signals are transmitted increases.
As dispersion takes place, the high and low thresholds are blurred. The more blurring that takes place, the more difficult it is to interpret data bits. Consequently, some of the data bits embedded in the signals may be erroneously interpreted. While it is expected that some erroneous interpretations of data bits may occur, most communication standards specify a maximum number of erroneous interpretations that may occur. This is usually specified as the maximum bit error rate. For example, the maximum bit error rate in 10 Gigabit Ethernet systems is 10−12. To the extent that dispersion causes more errors than a specified maximum bit error rate, the effects of the dispersion should be mitigated.
One attempt to mitigate the effects of dispersion involves using single mode fiber, such as a single mode fiber 110 shown in FIGS. 3 and 4. As shown in FIGS. 3 and 4, the single mode fiber 110 typically includes a core 112 surrounded by a cladding 114. Advantageously, the single mode fiber 110 is configured to transmit a signal directly down the single mode fiber 110, which helps to mitigate the effects of dispersion. Unfortunately, installing single mode fiber may be more difficult, more time consuming, and/or more expensive than installing multi-mode fiber. Further, replacing existing multi-mode fiber with single mode fiber may also be difficult, expensive and time consuming. Moreover, as data rates increase, dispersion may occur with single mode fiber.
One attempt to mitigate the effects of dispersion involves using electronic dispersion compensation circuits. Unfortunately, such circuits may apply electronic dispersion compensation solutions that can get stuck in an undesirable local minimum. For example, the electronic dispersion compensation circuits may incrementally change the electronic dispersion compensation solutions and then determine whether the incremental changes yield better or worse results. If a worse result occurs, the electronic dispersion compensation circuit may return to a prior solution. In some instances, the prior solution may be less than desirable, but the electronic dispersion compensation circuit may always return to the prior solution because the incremental changes yield worse results.
Dispersion may occur in optical-based communication systems and also in other communication systems. For example, dispersion may occur in electrical based communication systems, such as those using copper or other conductor-based transmission lines.